Make stronger or more durable (7)
I believe the answer is:
toughen
(Other definitions for toughen that I've seen before include "Increase stiffness" , "Make stronger and hardier" , "Harden, strengthen" , "Fortify" , "Make strong and durable" .)
I believe the answer is:
toughen
(Other definitions for toughen that I've seen before include "Increase stiffness" , "Make stronger and hardier" , "Harden, strengthen" , "Fortify" , "Make strong and durable" .)